The invention pertains to a method for the continuous casting of a metal strand in a continuous casting installation, in which the metal, which has been formed into a slab with a still molten core in a casting machine, is brought vertically out of a mold, wherein the slab is guided downstream of the mold in the conveying direction through a number of segments, wherein each segment comprises a number of segment rollers, which are configured to make contact with the surface of the slab, and wherein, in the area upstream of the end of the casting machine, a number of segment rollers are raised from the surface of the slab or are not installed in the mountings provided, so that the contact between the slab and the segment rollers is interrupted or not present. The invention also pertains to a continuous casting installation.
The production of a strand by a method of the class in question is sufficiently well known from the prior art. The cast strand, that is, the slab, leaves the mold with a core which is still in the molten state. Along the curved apron, the slab is deflected from the vertical to the horizontal, for which purpose a number of apron segments are used. Each segment of the curved apron has a number of segment rollers, which are arranged in pairs to contact the slab on opposite sides.
With respect to the prior art, reference is made to FIGS. 1-3. FIG. 1 shows a side view of a casting machine forming one component of a continuous casting installation. FIG. 2 shows the change in temperature between the mold and a furnace, located downstream of the casting machine.
It can be seen in FIG. 1 that the continuous casting installation 1 comprises the casting machine 2, which has a number—eight being shown in the present case—of apron segments 4, 5, 6, 7, 8, 9, 10, and 11, which form a curved apron 3. The mold and the first three apron segments are not shown. Along the curved apron 3, the cast slab is conveyed in the conveying direction F to the end 14 of the casting machine, during which process it is deflected from the vertical to the horizontal.
A number of pairs of segment rollers 12, 13 are supported in each apron segment 4, 5, 6, 7, 8, 9, 10, 11; the slab is conveyed between each pair.
The length of the casting machine (from the mold to the end 14 of the casting machine) is usually determined in such a way that, at maximum mass flow (corresponding to the thickness or cross section of the slab times the casting speed), the solidification of the cast strand occurs while the strand is still within the last apron segment (i.e., in the present case, in apron segment 11). The temperature curve resulting from this is shown in FIG. 2, based on the example of a 16.4-m-long curved apron installation. Shown are the core temperature TK, the surface temperature TO (on the bottom of the slab), and the mean value of the temperature TM over the slab thickness as the slab passes through the casting machine and reaches the downstream roller hearth furnace. The end 14 of the casting machine and the entrance 19 of the furnace are indicated.
The average outlet temperature of the thin slab emerging from the casting machine is greater than 1,200° C. in this case. As it travels toward the furnace, the slab loses another 70° C. or so to the free surroundings and to the rollers, etc. As a result of the high mass flow, however, the temperature level at the entrance to the furnace is still sufficiently high (here: 1,166° C.).
It should be mentioned that the complete solidification of the slab occurs shortly before the end 14 of the casting machine; this point is designated by the number 23.
The continuous casting installation, however, is not always operated under optimal conditions or at maximum casting speed. Depending on the product to be cast, furthermore, slower casting speeds may be required for reasons of casting technology (e.g., surface quality, crack prevention, casting stability). It can and must be possible to adjust the casting speed of the casting machine flexibly. Again for reasons of casting technology, however, the cooling of the strand cannot be adapted however one might wish to a lower mass flow. At lower mass flows, the casting strand therefore solidifies a good distance before the end of the continuous casting installation, as can be seen in FIG. 3. Here, again, the change in the temperature between the mold and the furnace is shown, but now at a slower casting speed in comparison to FIG. 2. The point at which the slab solidifies is again designated by the number 23 and is situated far upstream of the end 14 of the casting machine. After complete solidification, the strand loses an additional 150° C. or so in this example (see ΔT1) as it travels onward through the continuous casting installation before reaching the end 14 of the casting machine. Because of the low mass flow, the temperature loss between the end of the continuous casting machine 14 and the entrance 19 to the furnace is relatively high also (see ΔT2: approximately 100° C. in this example), so that, in the present case, the average temperature on entry into the furnace is often only about 987° C.
At a low mass flow, therefore, the slab loses a considerable amount of energy and solidifies quickly within the continuous casting installation as it is being transported to the furnace.
A method of the type indicated above is known from DE 76 13 430 U1. Additional solutions are disclosed in GB 1 603 428 A; WO 2007/137759 A1; WO 2007/073841A1; DE 10 2010 022 003 A1; and EP 0 287 021 A2.